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Make TypeFolder::fold_* return Result #432

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LeSeulArtichaut opened this issue May 17, 2021 · 3 comments
Closed
3 tasks done

Make TypeFolder::fold_* return Result #432

LeSeulArtichaut opened this issue May 17, 2021 · 3 comments
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major-change A proposal to make a major change to rustc major-change-accepted A major change proposal that was accepted T-compiler Add this label so rfcbot knows to poll the compiler team

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@LeSeulArtichaut
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LeSeulArtichaut commented May 17, 2021 •
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Proposal

Add a TypeFolder::Error associated type, and make all TypeFolder::fold_xxx methods return a Result<XXX, Self::Error>.
This gives more flexibility to implementors of the trait:

  • if they never fail, they can set Error = !
  • if they can fail, they have the choice of panicking, or propagate an Err, possibly holding a meaningful value

Implementation in rust-lang/rust#85469.

This is similar to #383, which added TypeVisitor::BreakTy.
This is also a part of the Traits WG's plan for a shared type library (cc rust-lang/types-team#16)

Example application

rust-lang/rust#85103: if we want to compute the layout of a type, we first need to normalize it, but normalization panics if it fails. One way to fix this is to make normalization, which uses TypeFolder, return a Result instead of panicking. This would be easy if this proposal was implemented.

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@jackh726

Process

The main points of the Major Change Process are as follows:

  • File an issue describing the proposal.
  • A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
    • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
    • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
  • Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

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This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.
@LeSeulArtichaut LeSeulArtichaut added T-compiler Add this label so rfcbot knows to poll the compiler team major-change A proposal to make a major change to rustc labels May 17, 2021
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rustbot commented May 17, 2021

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

@rustbot rustbot added the to-announce Announce this issue on triage meeting label May 17, 2021
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@rustbot second

@rustbot rustbot added the final-comment-period The FCP has started, most (if not all) team members are in agreement label May 17, 2021
@nikomatsakis nikomatsakis mentioned this issue May 18, 2021
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@LeSeulArtichaut LeSeulArtichaut mentioned this issue May 19, 2021
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@LeSeulArtichaut LeSeulArtichaut mentioned this issue May 19, 2021
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@apiraino apiraino removed the to-announce Announce this issue on triage meeting label May 20, 2021
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apiraino commented Jun 3, 2021

@rustbot label -final-comment-period +major-change-accepted

@apiraino apiraino closed this as completed Jun 3, 2021
@rustbot rustbot added major-change-accepted A major change proposal that was accepted to-announce Announce this issue on triage meeting and removed final-comment-period The FCP has started, most (if not all) team members are in agreement labels Jun 3, 2021
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Jun 4, 2021
@eggyal eggyal mentioned this issue Nov 25, 2021
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bors added a commit to rust-lang-ci/rust that referenced this issue Nov 28, 2021
@bors
Auto merge of rust-lang#91230 - eggyal:fallible-type-fold, r=jackh726
e6d2de9 
Make `TypeFolder::fold_*` return `Result`

Implements rust-lang/compiler-team#432.

Initially this is just a rebase of `@LeSeulArtichaut's` work in rust-lang#85469 (abandoned; see rust-lang#85485 (comment)).  At that time, it caused a regression in performance that required some further exploration... with this rebased PR bors can hopefully report some perf analysis from which we can investigate further (if the regression is indeed still present).

r? `@jackh726` cc `@nikomatsakis`
bors added a commit to rust-lang/chalk that referenced this issue Nov 28, 2021
@bors
Auto merge of #709 - LeSeulArtichaut:fallible-folder, r=jackh726
514a60a 
Introduce `Folder::Error`

Equivalent to rust-lang/rust#85469, cc rust-lang/compiler-team#432 rust-lang/types-team#16.

This compiles, but there are implementations of `Folder` that still use `Fallible<T>` instead of `Result<T, Self::Error>`. I can make that change if it is deemed beneficial.

r? `@jackh726`
@jackh726 jackh726 mentioned this issue Dec 1, 2021
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